Coal is an abundant source of energy. While coal is abundant, the burning of coal results in significant pollutants being released into the air. In fact, the burning of coal is a leading cause of smog, acid rain, global warning, and toxins in the air (Union of Concerned Scientists). In an average year, a single, typical coal plant generates 3.7 million tons of carbon dioxide (CO2), 10,000 tons of sulfur dioxide (SO2), 10,200 tons of nitric oxide (NOx), 720 tons of carbon monoxide (CO), 220 tons of volatile organic compounds, 225 pounds of arsenic and many other toxic metals, including mercury.
Emissions of NOx include nitric oxide (NO) and nitrogen dioxide (NO2). Free radicals of nitrogen (N2) and oxygen (O2) combine chemically primarily to form NO at high combustion temperatures. This thermal NOx tends to form even when nitrogen is removed from the fuel. When discharged to the air, emissions of NO oxidize to form NO2, which tends to accumulate excessively in many urban atmospheres. In sunlight, the NO2 reacts with volatile organic compounds to form ground level ozone, eye irritants and photochemical smog.
Exhaust-after-treatment techniques can reduce significantly NOx emissions levels using various chemical or catalytic methods. Such methods are known in the art and involve selective catalytic reduction (SCR) or selective noncatalytic reduction (SNCR). Such after-treatment methods typically require some type of molecular oxygen reductant, such as ammonia, urea (CH4N2O), or other nitrogenous agent, for removal of NOx emissions.
SCR uses a solid catalyst surface to convert NOx to N2. These solid catalysts are selective for NOx removal and do not reduce emissions of CO and unburned hydrocarbons. Large catalyst volumes are normally needed to maintain low levels of NOx and inhibit NH3 breakthrough. The catalyst activity depends on temperature and declines with use. Normal variations in catalyst activity are accommodated only by enlarging the volume of catalyst or limiting the range of combustion operation. Catalysts may require replacement prematurely due to sintering or poisoning when exposed to high levels of temperature or exhaust contaminants. Even under normal operating conditions, the SCR method requires a uniform distribution of NH3 relative to NOx in the exhaust gas. NOx emissions, however, are frequently distributed non-uniformly, so low levels of both NOx and NH3 breakthrough may be achieved only by controlling the distribution of injected NH3 or mixing the exhaust to a uniform NOx level.
SCR catalysts can have other catalytic effects that can undesirably alter flue gas chemistry for mercury capture. Sulfur dioxide (SO2) can be catalytically oxidized to sulfur trioxide, SO3, which is undesirable because it can cause problems with the operation of the boiler or the operation of air pollution control technologies, including the following: interferes with mercury capture on fly ash or with activated carbon sorbents downstream of the SCR; reacts with excess ammonia in the air preheater to form solid deposits that interfere with flue gas flow; and forms an ultrafine sulfuric acid aerosol, which is emitted out the stack.
SCR is performed typically between the boiler and air (pre) heater and, though effective in removing nitrogen oxides, represents a major retrofit for coal-fired power plants. SCR commonly requires a large catalytic surface and capital expenditure for ductwork, catalyst housing, and controls. Expensive catalysts must be periodically replaced, adding to ongoing operational costs.
Although SCR is capable of meeting regulatory NOx reduction limits, additional NOx removal prior to the SCR is desirable to reduce the amount of reagent ammonia introduced within the SCR, extend catalyst life and potentially reduce the catalyst surface area and activity required to achieve the final NOx control level. For systems without SCR installed, a NOx trim technology, such as SNCR, combined with retrofit combustion controls, such as low NOx burners and staged combustion, can be combined to achieve regulatory compliance.
SNCR is a retrofit NOx control technology in which ammonia or urea is injected post-combustion in a narrow temperature range of the flue path. SNCR can optimally remove up to 20 to 40% of NOx. It is normally applied as a NOx trim method, often in combination with other NOx control methods. It can be difficult to optimize for all combustion conditions and plant load. The success of SNCR for any plant is highly dependent on the degree of mixing and distribution that is possible in a limited temperature zone. Additionally, there can be maintenance problems with SNCR systems due to injection lance pluggage and failure.
Recent tax legislation provided incentives for reducing NOx emissions by treating the combustion fuel, rather than addressing the emissions through combustion modification or SNCR or SCR type technologies downstream. To qualify for the incentive, any additive must be added before the point of combustion. The goal does not provide a straight forward solution, as the traditional reagents used to treat NOx do not survive at combustion temperatures. Therefore, a compound is required that can be mixed with the combustion fuel, move through the combustion zone, and arrive in the post-combustion zone in sufficient quantity to measurably reduce NOx.